Epoxy to polyester

Discussion in 'Materials' started by valvebounce, Jun 7, 2012.

  1. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    I have a 13ft 60's speedboat which I have replaced the stringers in.I have not yet glassed them in.
    I was looking for materials to do the job when I was informed that poly resin does not adhere to poly resin hulls too well,and to use epoxy,as this does a better job.I also need to glass in a new deck,which is 3/4"marine ply coated with fibregass both sides.The hull on my boat is a single skin of about 5mm thickness.
    Any advice will be gladly accepted.
    I have also been offered aircraft quality epoxy paint at a very reasonable price,and as I plan a complete re-paint of the outside of the hull,would it be advisable to use it?The hull has a gelcoat on it,but it is badly crazed and cracked.I have removed a small area of the gelcoat with a heatgun and scraper,which has taken it down to the red undercoat beneath the gelcoat.
    The hull seems ok underneath where I removed the paint with the heatgun,is it advisable to use this method,or will it effect the strength of the hull?
     
  2. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    While you will get some feed back stating differently, poly bonds to poly quite well, not as good as epoxy, but repairs don't typically fail unless poorly done. So you can use either one.

    If the gel coat is crazed it means its in bad shape and should be removed if you want it to look nice long term. There are some products that say they will cover the crazing, but long term results may not be all that good. Crazing means the gel coat itself was poorly applied (many different possible problems) and will continue to deteriorate, it will just do under the coating you put over it causing it to fail too.

    Unless you have a two tone gel coat scheme, or the boat has been refinished before, there shouldn’t be a red layer under the gel coat, what’s under the gel coat should be a translucent dark amber or brown.

    3/4 ply is a little thick for such a small boat, you don't need or want the added weight.

    As for paint, without more info on what it is it's hard to say, so no opinion.
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Yep, polyester bonds to polyester pretty good, assuming sufficient tooth, though over wood, it's certainly not the best route, particularly compared to epoxy's performance in both bonding to wood and polyester.

    Most aircraft use LPU's not epoxy, though often they are primed with an epoxy. What is the brand and specific product name? I wouldn't use a heat gun to remove the gel coat. Just sand it. You'll get a start on fairing and smoothing this way too.

    Agreed, 3/4" plywood is 50% heavier then necessary on your boat. 1/2" will do fine and save at least a cooler full of beer in weight too.
     
  4. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Thanks for the replies gentlemen,they are much appreciated.
    The reason I used a heatgun was because even the coursest grade of grit paper [40 aluminium oxide]fills up and clogs after just a couple of minutes.
    The gelcoat is very soft,and rolls off with a little heat and a scraper.Like you say,it was probably a bad job in the first place.The paint or gelcoat stays tacky on the scraper when it is removed,and I have to use another scraper to remove it.The red primer-undercoat has a flat finish and I thought maybe
    it was a primer to enable the topcoat to grip the fibreglass hull.Its an old boat,with no history,and I think it has had a few "Bush mechanics"working on it over the years.The hull fibreglass is 5mm thick,and is a light blue colour throughout.I can only presume this is what they used here in Engand in the 60's.
    I have already purchased the deck ply,so it looks like I will have to forego that cooler full of beer PAR,Haha.
    I will be speaking to the guy with the paint over the weekend,so I will get the details and pass them on to you.He did say it is a two part epoxy paint,,and is used on aircraft because of its flexibility and durability.
    He has black,light and dark grey,international orange and brown.I think I might opt for black and add a few reflector strips.I think the grey is royal navy,so I could do without the camoflage if I have to call the coastguard or rescue services for some reason.
     
  5. ondarvr
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    ondarvr Senior Member

    It sounds more like there is a layer of paint over red gel coat.

    If the resin is blue, it would mean it's been tinted.

    Black is a terrible color for a boat, not only is it hard to keep clean, but it also gets very hot in the sun, often far too hot to touch.
     
  6. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    Ondarvr,Hi,
    I know he same sun shines on us both,but it doesnt try as hard here in the UK unfotunately.I hadnt thought about keeping it clean,which is a good point.Years ago I had a dark blue bathroom suite fitted,it looked pristine,but was a nightmare to keep from looking streaky,needless to say,it wasnt in long.
    I failed to mention that the boat is green under the topcoat in places,which can be seen under the cracks and crazing.The red is very thin,and is much like the dope coat they use on model aircraft,it doesnt melt or budge with applied heat.
    Like you said,applying a filler coat wont be any use,I think removing everything down to the red is the only way to obtain a decent finish.The boat just needs to look presentable,especially as it will have my name on the workmanship,if you know what I mean.
    Another point,which should probably be on a different part of the forum,I have never had or seen this boat in the water.When I got it there was no deck,just duckboards resting on the hull between the stringers.I have replaced the stringers,but i am at a bit of a loss as to the level of the deck from front to back.I suppose the deck should match the level of the waterline,although above it phsycally.The only thing I think I could use as a datum are the side strakes or bump strips that are still fitted.To get a level I will make feathering pieces to fit on the stringers,or cross pieces of different heights to follow the graduation on top of the fibreglassed stringers.
     
  7. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    make sure you grind fiberglass good before repair, repair in fiberglass. And if it is in budget, throw some epoxy on it to make sure you have everything seals up. Sometimes amateurs leave the fiberglass with holes or expose cloth. Either another layer of resin or epoxy will solve that problem.
     

  8. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Thanks mydauphin,I can see your point,it will probably be a great idea when I fit the new transom,I have removed the old one completely including the timber and fibreglass,all there is to fix to are the back corners and a 1"lip above the hull base.The method you suggest should deal with making it watertight,and add strength where the motor adds torque when in use.
     
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